Many different devices which, e.g., function on the basis of radar, lasers, or ultrasound, are used to detect objects, and in particular, to measure distance. Because of their very high resolution, ultrasonic sensors are especially useful in close range.
For example, such a device is described in German Published Patent Application No. 44 25 419, having a rear-side transmitting and receiving arrangement; a front-side transmitting and receiving arrangement, which includes at least one transmitting and receiving unit in the middle of the front area, and a transmitting and receiving unit in each of the front corner areas; a control unit for activating and deactivating the transmitting and receiving arrangement; and acoustic and/or optical warning elements, which generate warning signals as a function of the output signals of the transmitting and receiving arrangements, in response to the transmitting and receiving arrangements being activated; both the front-side and rear-side transmitting and receiving arrangements being activated as long as the reverse gear is active, and the vehicle speed does not exceed a first threshold value. On the other hand, only the front-side transmitting and receiving arrangement is activated as long as the reverse gear is not engaged and the driving speed does not exceed a predefined second threshold value. The two transmitting and receiving arrangements are deactivated in all other driving conditions.
In the case of motor vehicles, measuring distance using ultrasonic transducers as a parking assistant, or measuring distance to detect lateral obstacles is based on measuring the echo time of the sound. For this purpose, a transducer mounted on the motor vehicle emits a signal that is reflected by an obstacle. This echo can be received by additional transducers mounted on the motor vehicle. Either short wave trains or a continuous wave train provided with a fixed or random identifier are used as a transmitted signal. The echo time of the short wave train is directly determined from the time at which it arrives at the receiver. In the case of a continuous wave train, the echo time is measured by correlating the emitted and the received signals.
The problem of using such systems is that two vehicles can interfere with each other. In this case, false identifications of obstacles can result when the transmitted signal of the one motor vehicle is incorrectly interpreted by the other as an echo of its own transmitted signal. This can lead to a non-existing obstacle being detected, or worse, an obstacle not being detected because the weak echoes are masked by a strong interference signal of another motor vehicle, which may occur when the amplitude of the received signal is regulated and the setting time is in the order of magnitude of a measuring cycle. A strong interference signal causes a low amplification of the amplitude regulation device AGC so that a weak echo is not sufficiently amplified to detect an obstacle with certainty.
An additional problem occurs when two or more transducers on a vehicle transmit at the same time, in order to, e.g., increase the measuring rate. Even when the transmitting transducers are spatially separated in such a manner, their transmitted echoes are normally only received by different receiver groups. Thus, it cannot be ruled out that signal echoes reach an unintended receiver group, when an unfavorable arrangement of obstacles or an unfavorable orientation of their surfaces exists. This can also lead to false identification of obstacles. Solution options include detecting these mutual interferences (signal collision detection) or preventing these mutual interferences (signal collision prevention). Signal collision prevention has the advantage of the obstacle detection not being interrupted by another transmitter.
Therefore, it is one object of the present invention to provide a method and a device for detecting objects, in which other sources are prevented from influencing the measurement accuracy.